File handle leak with TCP connections in CLOSE_WAIT when using distributed builds

I traced this into the way the XMLRPC is set up.

Atlassian XMLRPC Binder sets up Apache XMLRPC Client using the Commons HTTP Client 3.1 transport. HTTP Client is using the default SimpleHttpConnectionManager. These are all hard coded.

Because the binder is not reused, a new XMLRPC client is set up for each call. The XMLRPC client calls releaseConnection on the HTTP Client, which leaves the physical connection open for reuse - but with nothing set up to reuse it or terminate it, it is left open until GC happens on the socket. This is often quick, but some connections make it into the more permanent area of heap.

See: http://fuyun.org/2009/09/connection-close-in-httpclient/

Medium term, the XMLRPC should be replaced with JAX-RS.

Potential short term solutions:

1. adjust the binder to use the JDK URL Connection instead of the HTTP Client
2. try and adjust the HTTP Client to use a multi-threaded connection manager, and then reuse the binder/xmlrpc-client/http-client
3. configure HTTP Client's background job to cleanup idle connections

This needs further investigation.

went with option #2. The JDK URL connection could well have had other impacts.

While just setting the connection manager to close after each request, went with the multi-threaded manager which should manage the connections efficiently, even if a couple remain open.

Note that there are other occasions where sockets are left open (e.g. AbstractContinuumProjectBuilder, which has its own httpclient v4). Unifying the HTTP clients used would be beneficial in future, but is best evaluated if the XMLRPC layer gets replaced with something else.